1. Field of the Invention
The present invention relates to a fuel assembly which is used in a pressurized water reactor.
2. Description of Related Art
A conventional fuel assembly used in a pressurized water reactor, which includes, a lower nozzle disposed on a lower core plate of the pressurized water reactor, an upper nozzle having a hold-down spring for holding down the lower nozzle against the lower core plate, a plurality of control rod guide thimbles for guiding control rods extending through the upper nozzle toward the lower core plate, a plurality of supporting grids mounted on the control rod guide thimbles, and a number of fuel rods held by the supporting grids in parallel with the control rod guide thimbles, is disclosed in a pamphlet published by FRAMATOME Co. entitled xe2x80x9cSNE97-La Coruna-Nov. 5-7, 1997xe2x80x9d.
In a fuel assembly such as mentioned above, when the control rods, detached from the control rod driving unit upon emergency shutdown of the nuclear reactor, fall within the control rod guide thimbles without being decelerated, the upper nozzle of the fuel assembly may be subjected to an excessively large impact force. Consequently, dashpots of a thin-tube configuration are formed within the control rod guide thimbles so that the speed at which the control rods fall within the control rod guide thimbles is decreased to thereby dampen the impact force applied to the upper nozzle.
However, in a conventional fuel assembly such as described above, the dashpot has a length ranging from 0.16 L to 0.18 L, wherein L represents the entire length of a control rod guide thimble. Consequently, flexural deformation may take place in the dashpot under compression loads acting in the axial direction of the control rod guide thimble, giving rise to problems in that insatiability of the control rod may be impaired due to the flexural deformation of the dashpot.
In the light of the state of the art described above, it is an object of the present invention to provide a fuel assembly of a structure such that the dashpot can be positively protected against flexural deformation under the compression loads acting in the axial direction of the control rod guide thimble.
In view of the above and other objects, which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a fuel assembly for a pressurized water reactor which includes, a lower nozzle disposed on a lower core plate of the pressurized water reactor, an upper nozzle having hold-down spring means for holding down the lower nozzle against the lower core plate, a plurality of control rod guide thimbles for guiding control rods extending through the upper nozzle toward the lower core plate, a plurality of supporting grids mounted on the control rod guide thimbles, a number of fuel rods held by the supporting grids in parallel with the control rod guide thimbles, and dashpots each formed in a thin tubular shape in each of the control rod guide thimbles for reducing thee falling speed of the control rods. Each dashpot is comprised of a large diameter section which has a diameter substantially equal to that of the control rod guide thimble and which is formed at a lower portion of the dashpot and a small diameter section which has a diameter smaller than that of the large diameter section and which is formed at an upper portion of the dashpot, wherein the small diameter section has an effective length selected so as to fall within a range of 0.03 L to 0.1 L, where L represents a length of the control rod guide thimble.
In a preferred mode of the present invention, the effective length of the small diameter section may be selected so as to fall within a range of from 0.04 L to 0.06 L.
In another preferred mode, the length of the dashpot may be dimensioned so as to fall within a range of from 0.16 L to 0.18 L, while the length of the large diameter section may be so selected as to fall within a range of from 0.06 L to 0.15 L and more preferably within a range of from 0.1 L to 0.14 L In yet another preferred mode for carrying out the invention, another small diameter section may be additionally provided in a lower end portion of the large diameter section.
The above and other objects, features and attendant advantages of the present invention will be more easily understood by reading the following description of the preferred embodiments, to be taken only by way of example, in conjunction with the accompanying drawings.